I was in Hermosillo yesterday and today. I brought this up and people said it was simple. Get a permit from transito or check the book online to know the rules. I told them it was in Spanish and they said, "well then, that's too bad".
What would they say to a Mexican who is traveling in the U.S. and doesn't know the rules? Would the policeman just let them go? Best not to answer because you know they wouldn't. Remember Big Wells, Tx where the sheriff's department accepted and still does, mordida.
I'm just telling you how a group of 50 Mexican teachers from Hermosillo answered the question. :)